1. Field of the Invention
The present invention is directed to a process of forming a curved profile on a semiconductor device, particularly by anodization of the substrate.
2. Description of the Related Art
One prior art is disclosed in Japanese patent publication No. 55-13960 which is directed to a process of making a micro structure in the surface of a semiconductor substrate by anodization. The anodization is used for oxidizing the top surface of the substrate in an electrolyte solution. The oxidizing proceeds selectively in the top surface in a pattern corresponding to an arrangement of an anode which is formed separately from the substrate and is held in contact with the bottom surface of the substrate, leaving oxidized portions partially in the top surface of the substrate. Thereafter, the oxidized portions are removed to leave a convex in the top surface of the substrate. This technique is sufficient for forming a relatively thin profile or surface irregularity with a small depth. However, it is found practically difficult to provide a thick profile with a smoothly curved surface as is required in many application fields, since the oxidized portions act as a dielectric barrier blocking an anodizing current through the substrate to inhibit the growth of the oxidized portions in the thickness of the substrate. Accordingly, in order to obtain the thick profile with an accurately designed curved contour, it has to be required not only to repeat steps of anodizing the top surface to partially form the oxidized portions followed by removing the oxidized portions, but also to use differently arranged anodes in each step. In this sense, the above prior art is found not to be available for providing a curved surface having sufficient depth or thickness.
Another prior art is disclosed in Japanese patent publication No. 2000-263556 which is directed to a process of making a mold for a micro optical lens. The mold is fabricated by steps of preparing a semiconductor substrate, providing a dielectric mask on a top surface of the substrate, forming one or more openings in the mask, placing the substrate in an electrolytic solution, and anodizing a portion in the top surface not covered by the mask to convert the portion into a porous zone. Thereafter, the porous zone is removed to leave a rounded convex in the top surface of the substrate. An ultraviolet curable resin is placed in the convex and is cured therein to obtain a convex lens. Although the prior art discloses the formation of the porous zones which does not impede the anodization, the process relies upon the mask with the opening so that the porous zone develops isotropically from the center of each opening. Accordingly, the resulting rounded convex is limited to have substantially a uniform radius of curvature. With this limitation, the process cannot be not adapted to realizing a curved profile having non-uniform radius of curvature or sophisticated surface profile.
Further, when forming a small curved surface configuration by use of the mask with tiny opening, the porous layer can be formed successfully around the tiny opening at an initial stage. However, bubbles appearing in the initial stage are difficult to escape out through the tiny opening and are likely to remain in the porous zones, which inhibits the entry of the electrolyte solution in the substrate and therefore impedes further development of the porous zone. With this result, the porous zones could not be controlled to give an intended predetermined profile, failing to reproduce the accurate profile. When, on the other hand, forming a relatively large surface profile with the use of the mask having a correspondingly large opening, it is likely that the anodization proceeds in the center of the opening at a rate considerably faster than at the periphery of the opening and the rate is difficult to be controlled. Therefore, it is difficult to give an accurately controlled surface profile to the porous zone and the resulting curved surface.
Moreover, because of that the mask is deposited on the side of the substrate from which the anodization proceeds, and the that the mask is normally made of SiN or the like having a relatively small thickness, for example, 1 μm or less, the mask may be easily broken due to a stress developing as the porous zone grows. Also in this respect, the process relying upon the mask on the anodization side of the substrate is not found satisfactory in providing the accurately controlled surface profile.